When ethylene oxide is prepared by the silver catalyzed, vapor phase, partial oxidation of ethylene with molecular oxygen, a gaseous reaction effluent is obtained. This effluent is extremely dilute with respect to the desired ethylene oxide product, normally containing only from about 0.3 mole % to about 5 mole % of this desired material. Recovery of the ethylene oxide from this effluent gas, as conventionally practiced, involves an initial water absorption step, followed by a stripping step, which is in turn followed by a reabsorption step. See, for example, U.S. Pat. No. 3,418,338.
In such a sequence, the function of the absorption step is to selectively absorb the ethylene oxide from the gaseous reaction effluent with but minimal concurrent absorption of other materials such as unconverted reactants, reaction diluents and reaction by-products which are also present in the reaction effluent. However, as the absorber operates at relatively high pressure, only slightly below that of the reactor, substantial amounts of such materials present in the reaction effluent as carbon dioxide, and any trace impurities such as aldehydic and acidic by-products formed during the oxidation reaction, are concurrently absorbed with the ethylene oxide. In the stripper the absorbed ethylene oxide is re-volatilized by steam stripping as also are substantial amounts of the carbon dioxide and the aldehydic trace impurities originally co-absorbed with the ethylene oxide. The function of the reabsorber is to separate between the ethylene oxide and the carbon dioxide volatilized within the stripper. Thus, within the reabsorber, the stripper overhead is countercurrently contacted with water to obtain, as a bottoms product, a liquid reabsorbate consisting essentially of ethylene oxide and water but also containing residual quantities of carbon dioxide, aldehydic and acidic trace impurities.
As indicated, the absorber operates at relatively high pressure, while both the stripper and reabsorber operate at pressures which are relatively close to atmospheric. This pressure differential facilitates the separations desired in the stripper-reabsorption system, primarily the separation between ethylene oxide and carbon dioxide.
However, this system is possessed of some disadvantages accentuated by the need for obtaining over higher purity products at minimum cost. Temperatures within the stripper are high enough to thermally hydrate a portion of the ethylene oxide to ethylene glycol which, being contaminated by trace impurities, is difficult to upgrade for uses requiring extremely high purity products such as fibers, or even to those requiring lesser purities, such as anti-freeze. Attempts to lower stripper pressure and thus attain lower temperatures within the stripper are generally unattractive since this also lowers pressure within the reabsorber unless vapor compression facilities are provided between the stripper and the reabsorber. Compression facilities have been found to be expensive, difficult to maintain and potentially hazardous. Without compression, however, the lower the pressure within the stripper, the lower will be the pressure within the reabsorber and the greater will be the quantity of water required to preferentially absorb the ethylene oxide within the reabsorber. This, in turn, means that the reabsorber bottoms product (hereinafter referred to as the "reabsorbate"-- a solution of ethylene oxide in water) becomes more dilute with respect to ethylene oxide; consequently, subsequent processing to recover ethylene oxide therefrom becomes more expensive and difficult.
Even when the reabsorbate is not to be processed for the recovery of ethylene oxide but rather is to be subjected to thermal hydration for conversion of the ethylene oxide dissolved therein directly to monoethylene glycol (with concomitant formation of higher glycols), the prior art systems present certain problems. The trace impurities dissolved in the reabsorbate interfere with glycol quality. Secondly, the economics of glycol production from the reabsorbate strongly favor the use of solutions relatively concentrated with respect to dissolved ethylene oxide since reabsorbate solutions containing less than, say, 5% by weight of ethylene oxide are difficult to economically employ in glycol production.
These problems have been further accentuated by recent developments in the oxidation reaction. These recent developments entail the use of large concentrations of carbon dioxide as a reaction diluent in the oxidation step (see Belgian patent No. 781,107). Correspondingly, a greater proportion of carbon dioxide is absorbed in the absorber, stripped in the stripper and thus is present in the feed gas to the reabsorber. In turn, this necessitates a substantially increased water rate to the reabsorber and results in a reabsorbate solution more dilute with respect to ethylene oxide.
The art is therefore faced with a need for a stripper-reabsorber system for use in an ethylene oxide recovery process which is readily operated, causes minimal glycol formation and which gives minimum carryover of trace impurities. Additionally, to take advantage of recent developments in the conduct of the oxidation reaction, such a system should be capable of processing materials having high carbon dioxide contents without concomitantly giving an aqueous reabsorbate solution excessively dilute with respect to ethylene oxide. Such a system is provided by this invention.